1. Field of the Invention
The present invention relates to a setters and a process for firing green ceramic honeycomb structural bodies by using these setters.
2. Related Art Statement
In the production of ceramic articles by firing green moldings, there has been heretofore known a method of firing the green moldings on plankings or burning tools called "setters", without directly being placed on shelves, to prevent the ceramic articles from cracking or attaching to the shelves.
In order to produce ceramic honeycomb structural bodies by firing green structural bodies thereof, there have been available a method (laid posture firing) in which the green honeycomb structural bodies are fired while axes of their cells are almost horizontally oriented and a method (erected posture firing) in which the green honeycomb structural bodies are fired while the axes of the cells are approximately vertically oriented.
In the production of the ceramic honeycomb structural bodies, the honeycomb structural bodies shrink or expand in a cell axial direction and in a direction orthogonal thereto during firing. Further, it may happen that the ceramic honeycomb structural bodies deform during firing. Therefore, it may be said that the erected posture firing in which a green honeycomb structural body is fired while its open lower end is contacted with an upper face of a setter is advantageous to obtain ceramic honeycomb structural bodies having excellent dimensional accuracy, because the deformation can be suppressed to a small extent during firing.
When a green honeycomb structural body is fired in the state that its open lower end face is contacted with the upper face of the setter, there is a problem that ribs of the cells of the green honeycomb structural body are cracked by a frictional resistive force developed at a contact interface between the green honeycomb structural body and the setter due to the shrinkage or the expansion during firing. This problem has become conspicuous with a recent rib-thinning tendency. In particular, as shown in FIGS. 1 and 2, when a green honeycomb structural body 7 is placed on a setter 1 which has an end shape substantially equal to that of the green honeycomb structural body and is put on a shelf 9, the outer edge portion of the green honeycomb structural body is liable to crack because of the frictional resistance discussed above. In addition, in the case of the erected posture firing, the lower end face portion and the central portion of the green honeycomb structural body near the setter are insufficiently heated. Thus, it is difficult to increase the temperature of these portions. When the green honeycomb structural body contains a binder, a foaming agent (which is added to bodies and burnt out to form pores in fired articles, for instance, graphite, wheat powder, brown coal, lime, charcoal, saw dusts, or cork powder), etc., they remain unburnt in the honeycomb structural body or the unburnt binder or foaming agent abnormally burns when the honeycomb structural body reaches a high temperature range, thereby producing cracks. When the unburnt binder, foaming agent, etc. is abruptly burnt in the honeycomb structural body, the following troubles occur.
(1) Since the inside of the honeycomb structural body expands, cracks are formed therein. PA0 (2) Since the inside of the honeycomb structural body is abruptly heated, the inside melts. PA0 (3) Since the inside of the honeycomb structural body is abruptly heated, the porosity and the pore diameter become different between the inner portion and the outer portion of the honeycomb structural body. PA0 (1) Since a gas having a higher temperature generally tends to collect upwardly, the temperature of the green honeycomb structural body is likely to become higher at the upper end face side (upward side) than at the lower end face side (downward side) (The upper end face side is likely to be more heated). PA0 (2) Since the lower end face side contacts with the shelf or the setter, it is not exposed to a combustion gas. To the contrary, the temperature of the upper end face is likely to become higher due to its exposure to the combustion gas (The upper end face is more likely to be heated).
Further, the present inventors have noticed that there were the following problems in the erected posture firing:
Therefore, the temperature may differ between the upper and lower sides of the ceramic honeycomb structural body.
If the temperature distribution is non-uniform as mentioned above, the firing shrinkage becomes non-uniform between the upper and lower side portions. As a result, the upper end face which is likely to be more heated is liable to crack. Alternatively, if non-uniform temperature distribution occurs during burning-out of the binder, the foaming agent, etc. contained in the green honeycomb structural body, an overheated portion abnormally expands. Consequently, there is a problem that the upper end face which is likely to be more heated is liable to crack.
Even if these problems attempt to be coped with by adjusting a kiln temperature distribution, gas stream, etc., the temperature distribution is difficult to control when the size of the green honeycomb structural bodies is large or when a large number of green honeycomb structural bodies are fired as in the case of a tunnel kiln. If the heating rate of the green honeycomb structural bodies is lowered to cope with the above problems, productivity becomes lower.
With respect to the laid posture firing, if the end face of the green honeycomb structural body is exposed to a combustion gas and is overheated and accordingly temperature locally rises, the temperature distribution may become non-uniform. If the temperature distribution is non-uniform, the end face which is likely to be more heated is liable to crack for the same reason as given in the erected posture firing.